linux-kernel - Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline

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Date:   Wed, 1 Jul 2020 15:34:42 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Christopher Lameter <cl@...ux.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline

* Michal Hocko <mhocko@...nel.org> [2020-07-01 10:42:00]:

> 
> > 
> > 2. Also existence of dummy node also leads to inconsistent information. The
> > number of online nodes is inconsistent with the information in the
> > device-tree and resource-dump
> > 
> > 3. When the dummy node is present, single node non-Numa systems end up showing
> > up as NUMA systems and numa_balancing gets enabled. This will mean we take
> > the hit from the unnecessary numa hinting faults.
> 
> I have to say that I dislike the node online/offline state and directly
> exporting that to the userspace. Users should only care whether the node
> has memory/cpus. Numa nodes can be online without any memory. Just
> offline all the present memory blocks but do not physically hot remove
> them and you are in the same situation. If users are confused by an
> output of tools like numactl -H then those could be updated and hide
> nodes without any memory&cpus.
> 
> The autonuma problem sounds interesting but again this patch doesn't
> really solve the underlying problem because I strongly suspect that the
> problem is still there when a numa node gets all its memory offline as
> mentioned above.
> 
> While I completely agree that making node 0 special is wrong, I have
> still hard time to review this very simply looking patch because all the
> numa initialization is so spread around that this might just blow up
> at unexpected places. IIRC we have discussed testing in the previous
> version and David has provided a way to emulate these configurations
> on x86. Did you manage to use those instruction for additional testing
> on other than ppc architectures?
> 

I have tried all the steps that David mentioned and reported back at
https://lore.kernel.org/lkml/20200511174731.GD1961@linux.vnet.ibm.com/t/#u

As a summary, David's steps are still not creating a memoryless/cpuless on
x86 VM. I have tried booting with Numa/non-numa on all the x86 machines that
I could get to.

-- 
Thanks and Regards
Srikar Dronamraju